netdev - Re: [PATCH] net: bridge: add max_fdb

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Message-ID: <9125CC0D-4B56-4797-8416-DA70DEF2914F@cumulusnetworks.com>
Date:   Thu, 16 Nov 2017 21:36:20 +0200
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Andrew Lunn <andrew@...n.ch>, Sarah Newman <srn@...mr.com>
CC:     Willy Tarreau <w@....eu>, netdev@...r.kernel.org,
        roopa <roopa@...ulusnetworks.com>
Subject: Re: [PATCH] net: bridge: add max_fdb_count

On 16 November 2017 21:23:25 EET, Andrew Lunn <andrew@...n.ch> wrote:
>> Linux bridges can also be used in small embedded devices. With no
>limit,
>> the likely result from those devices being attacked is the device
>gets
>> thrown away for being unreliable.
>
>Hi Sarah
>
>Just to get a gut feeling...
>
>struct net_bridge_fdb_entry is 40 bytes.
>
>My WiFi access point which is also a 5 port bridge, currently has 97MB
>free RAM. That is space for about 2.5M FDB entries. So even Roopa's
>128K is not really a problem, in terms of memory.
>
>> Maybe what's needed is two thresholds, one for warning and one for
>enforcement.
>> The warning limit would need to be low enough that the information
>had a good chance
>> of being logged before the system was under too much load to be able
>to convey
>> that information. The enforcement limit could be left as default
>inactive until
>> shown that it needed to be otherwise.
>
>What exactly is the problem here? Does the DoS exhaust memory, or does
>the hashing algorithm not scale?

Just a note - when net-next opens I'll send patches
which move the fdb to a resizeable hashtable that scales nicely even with hundreds of thousands of entries so only the memory issue will remain.

>
>It is more work, but the table could be more closely tied to the
>memory management code. When memory is getting low, callbacks are made
>asking to free up memory. Register such a callback and throw away part
>of the table when memory is getting low. There is then no need to
>limit the size, but print a rate limited warning when asked to reduce
>the size.
>
>    Andrew


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